The present invention relates to a method and apparatus for filling bags.
Fertilizers have recently have been distributed in large flexible walled containers which contain from 250 to 2000 Kgs or more of granular material. Typically, such containers comprise a woven or similar load-bearing outer bag, preferably incorporating lifting loops or other means by which the container can be lifted and handled before, during and after filling, and a moisture barrier inner liner within the load-bearing outer bag, within which liner the granular material is held and protected against the environment during transport and storage. Typically, the load-bearing outer bag is formed from one or more plies of a woven polyalkylene fiber and lifting loops are formed integrally at the open top of the bag, for example as described in European Patent No. 0118112. The inner liner is typically formed from a substantially water-impervious polyalkylene or polyvinylic sheet material which provides a weather-resistant envelope for the contents, the liner being supported by the outer load-bearing bag during lifting and handling of the container. Such a container is filled through the open top of the liner or through a filling tube formed as a narrow axial extension of the liner, and the container is closed by heat sealing or tying off the open top of the filling tube of the liner.
Many forms of such container have been proposed, and for convenience, the term FIBC will be used herein to denote such containers in general. Furthermore, such FIBCs find use in the transport and storage of a wide range of particulate materials, for example cement, sugar, polymer beads and fertilizers. For convenience, the invention will be described hereinafter in terms of granular fertilizers having a predominant particle size in the range 1 to 4 mm. While FIBCs provide a cost effective means for storing and transporting fertilizers in comparatively large unit loads, typically from 500 to 1500 Kgs, problems are encountered in filling such containers. In view of the large tonnages which have to be handled in a commercial bag filling operation, it is desirable to fill the FIBC as rapidly as possible, typically within a total time span of less than 10 seconds so as to introduce the minimum of interruption in the flow of the FIBCs from introduction to the filling station to output of the filled and sealed bags. The conventional methods for filling small bags containing typically 50 Kgs have not proved feasible.
A fundamental problem resides in the fact that the filling tube or open end of the liner through which the material flows must be comparatively narrow, since it is difficult to achieve rapid heat sealing of the tube or open end if it extends for more than about 30 to 50 cms. However, in filling the FIBC, air is displaced from the liner and must escape through the filling tube in the opposite direction to the incoming particulate material. This problem is aggravated if the liner of the FIBC has been pre-inflated with air, as proposed in GB A 1475019, to distend the liner prior to filling. Therefore, if the filling tube is narrow to reduce problems in sealing the neck, problems will arise in securing outflow of air from the liner as the air is displaced by incoming particles. As a result, it has been accepted that the speed of filling an FIBC is a balance between these conflicting needs, and the limiting factor is the need to release air from the liner during filling.
It has been found that if the stream of the in-flowing particulate material is caused to adopt a free falling substantially laminar flow through the filling tube of the FIBC, typically in the form a jet type of flow of the material, so as to form a gap, preferably an annular gap, between the stream of particulate material and the wall of the filling tube over substantially the length of the filling tube, this gap surprisingly permits the air to escape from the interior of the liner without significantly disturbing the free falling flow of the particulate material. As a result, air can vent freely from the liner as it is filled and the flow of the particulate material is not significantly impeded or disrupted by the escaping air, whereby the ideal free fall flow of the particles can be maintained and the maximum filling rate of the FIBC can be achieved. It has further been found that the filling tube can be narrower than has hitherto been considered necessary without affecting this laminar flow, so that heat sealing of the tube can be carried out more readily.